1. Field of the Invention
This invention relates generally to semiconductor Integrated Circuit (IC) package manufacturing, and more particularly, to a process for manufacturing Printed Circuit (PC) boards, wherein electrical interconnections are imbedded within the boards during their manufacture, thereby eliminating the need for mechanical drilling.
2. Description of the Related Art
It is desirable to provide electrical interconnection between different layers of an IC package. These electrical connections, or vias, are usually formed during the manufacturing process by mechanical drilling.
To manufacture a PC board, strands of fibers (e.g., glass) are woven together to form a sheet of cloth. This cloth is then dipped in resin and thermally cured forming a cured board core. Copper sheets or other conductive materials are then plated or otherwise formed on the board core. The copper sheets are eventually etched into circuit paths for electronic circuitry to form the PC board. Vias are commonly created by mechanically drilling holes through the copper sheets and board core, then plating conductive material along the side-walls of the drilled through-holes. This process is performed after the board core is cured.
The vias formed by mechanical drilling are the single most time consuming and expensive process step in IC package manufacturing. Cracks can occur during normal operation due to stresses produced by temperature cycling. These cracks are usually caused by poor drilling that results in uneven via side-walls. Also, the combination of materials used in the manufacture of a PC board (e.g., glass fibers and resin) is difficult to drill through smoothly and cleanly after curing.
Furthermore, to make IC packages smaller with increased density, the mechanically-drilled vias must be reduced in size. Unfortunately, the size of a drilled via is limited by the mechanical limitation of the drill-bit diameter. Ultra-small drill-bits are expensive to manufacture and the useful life is limited. Moreover, sufficient plating of the via side-walls becomes more difficult as the aspect ratio (i.e., the ratio between a via diameter and its length) increases.
Accordingly, there remains a need for a process by which electrical connections may be made between layers of an IC package without resorting to mechanical drilling.